Using a newly developed method both for estimating relative levels of IgG1-class and IgG2-class antibodies of a given specificity in a serum, and for converting antisera or globulins to preparations containing only the IgG2 class or IgG1 class among the S7 antibodies of that specificity, it is planned: (a) To confirm our recent evidence suggesting that anti-graft strain antibodies of IgG2-class can cause rejection of skin allografts in the mouse (and anti-graft strain antibodies of IgG1-class, enhancement). (b) To examine the relation of these observations to the course of events which might occur in normal allograft rejection. (c) To study other biological aspects of graft rejection by anti-graft strain antibody of IgG2-class, including the effects of such antibodies on grafts of other tissues or organs, such as heart muscle implants; attempts at rejection of skin allografts in the "B mouse" (thymectomized, irradiated, bone marrow injected); experiments between congenic strains; and the histologic picture of rejection caused by alloantibody. (d) To study the relative affinities of the two competing antibodies and their relative efficiencies in the respective biologic functions. (e) To explore the possibilities of immunizing allogeneic mice with histocompatibility antigens in ways which would increase the ratio of IgG1 to IgG2 among the alloantibodies produced.